ICMCTF 2025 Session MA2-2-WeM: Thermal and Environmental Barrier Coatings II

The development of protective coatings is crucial for mitigating the severe corrosion caused by molten salts operating at high temperatures in Concentrating Solar Power plants. While nitrate-based salts are currently used as thermal storage media in CSP plants, carbonate salts offer superior thermal stability. The properties of the Li-Na-K carbonate eutectic were enhanced by the addition of 0.5% of α-Al₂O₃ nanoparticles. The new mixture presents a reduced melting point and an increase in the decomposition temperature, allowing the operation of the CSP plants at 700 °C. This increase would enhance steam generation efficiency and would reduce the Levelized Cost of Electricity (LCoE). However, the intense corrosive effects of molten carbonates demand the development of durable protective coatings to extend the lifespan of critical components.

This study evaluates the performance of yttria-stabilized zirconia (YSZ) sol-gel coatings applied to the 310H austenitic steel when it is exposed to static and dynamic conditions in a lab-scale setup. Corrosion tests were conducted at 700°C for up to 2500 hours in the eutectic ternary Li₂CO₃–Na₂CO₃–K₂CO₃ mixture with α-Al₂O₃ additive. The protective behavior of the coatings was assessed through gravimetric analysis, microstructural characterization and XRD. The performance of the coated material was also compared with the uncoated substrate. The coated samples exhibited an improved corrosion resistance, whereas the uncoated steels showed substantial degradation, including detachment and high mass variation. The presence of the YSZ coating reduced the corrosion extent by over one-third, with the thickness of the corrosion products measuring approximately 200 μm - 300 μm on the uncoated substrate, and between 50 μm - 70 μm on the coated samples.

The YSZ coating exhibited very similar behavior under both dynamic and static conditions. No significant difference was observed in the thickness of the corrosion product layer. Additionally, the same multilayer structure was identified, with the outer layer mainly composed of LiFeO₂, while the inner region was enriched in iron and chromium oxides. However, the uncoated austenitic steel showed higher degradation when exposed to dynamic conditions, displaying cracks along the corrosion layer and detachments.

These findings highlight the potential of YSZ coatings to enhance the durability of structural materials in next-generation CSP plants employing molten carbonates.

High-entropy alloys exhibit various properties, such as superior thermal stability, oxidation resistance, and corrosion resistance. These characteristics have sparked interest in using HEAs as anti-oxidation protective coatings. The nanotwinned structure within these alloys contributes to their high hardness and thermal stability, while the sluggish diffusion in high entropy alloys helps lower oxidation rates.